1. Field of the Invention
The present invention relates to an electric double-layer capacitor, and a process for producing the same.
2. Description of the Related Art
An electric double-layer capacitor is conventionally put to practical use as a capacitor in which a high capacity on the order of farad (F) can be provided without being accompanied by a chemical reaction and which is strong to the charging and discharging of large electric current and to a charging/discharging cycle.
In recent years, a new application of such an electric double-layer capacitor as an auxiliary power source or the like of a small-sized electronic device such as a mobile phone and the like by utilizing the feature of the electric double-layer capacitor. A further reduction in size and a further increase in capacity of the electric double-layer capacitor are required.
Such an electric double-layer capacitor includes a basic cell comprising a pair of collectors, a separator disposed between the collectors, and a pair of polarizable electrodes each of which is disposed between each of the collectors and the separator and in which an electrolytic solution is contained. The collectors provided at opposite ends of a laminated-cell structure made by laminating the basic cells in series with the basic cells or their collectors as connections are electrically connected to electrode plates included in lead terminal/electrode plate assemblies through conductive bond layers, so that they are charged from and discharged to the outside.
The capacity of the cell structure can be changed easily by changing the number of the basic cells laminated and connected in series and hence, the electric double-layer capacitor is comprised of the laminated-cell structure in many cases rather than the single basic cell.
The basic cell can be produced using a producing process disclosed, for example, in U.S. Pat. No. 6,377,441 by Ohya and others.
The collector has functions to encapsulate the electrolytic solution and to provide an electric conductivity, and may be comprised a matrix formed of an electrically insulating resin and conductive particles dispersed in the matrix in many cases. In general, materials disclosed in PCT/JP98/01021 by Tsukakoshi and others and in PCT/JP98/03073 by Akita and others can be used as a resin material suitable for the collector. The thickness of the collector is conventionally about 0.2 mm, but in recent years, a thinner film having a thickness of several tens xcexcm has been started to be used in order to meet the need for a reduction in size of the components.
The use of the electric double-layer capacitor as a variety of small-sized electronic devices such as an auxiliary power source for a mobile phone and the like is expected as described above, but an electric double-layer capacitor existing presently suffers from the following two problems in respect of equivalent series resistance (which will be referred to as ESR hereinafter) of a product, i.e., an initial inferiority and a low long-term reliability.
(1) There are many products having an initial ESR larger than a practical level. The ESR value is dispersed largely among products, resulting in a degraded yield.
(2) If a product having a small initial ESR has been put in service for a long period, its ESR is increased remarkably and for this reason, such product is not effective in service.
In a case of a thin collector film, particularly, it is difficult to ensure the long-term reliability of ESR, which is an obstacle to the real employment of the electric double-layer capacitor to an application expected.
The capacitor is used in wider applications such as an integrated circuit, a backup power source, a coupling circuit, a high-frequency circuit and the like, but the ESR is an important factor governing the overall characteristic in each of the applications. Therefore, it is desired that the ESR value is extremely small and stabilized. Particularly, when the electric double-layer capacitor is used in an application as a power source, a larger capacity and a lower ESR value are considered as ideal.
The ESR is governed mainly by a loss resistance of a dielectric material in a lower frequency range and mainly by a resistance of the component itself constituting the capacitor and a resistance between the components in a higher frequency range. What attention is paid to in the present patent application is particularly factors associated in the higher frequency range, which include
(a) a resistance of the collector itself (material),
(b) a resistance of the electrode plate itself (material),
(c) a state of contact between the basic cells (between the collectors), and
(d) a state of contact between the outermost collector of the cell structure and the electrode plate.
In order to improve (c) and (d) of these factors, it has been proposed to roughen the surface of the collector which is electrically bonded to the other collector or the electrode plate through the conductive bond layer so as to increase the contact area thereof.
The collector comprises the resin matrix and the conductive particles, as described above, and electrical conduction paths are defined between an interface between the collectors or between the collector and the electrode plate and an interface between the collector and the polarizable electrode. Each of the electrical conduction paths is formed by the conductive particles connected in a row in such a manner that some of those of the conductive particles at opposite ends of the conductive particles connected in the row are exposed to the surface connected to the other collector or the electrode plate and the surface opposed to the polarizable electrode.
However, the density of conductive particles dispersed in the collector is limited for the viewpoint of a producing process and hence, the number, per unit area, of the conductive particles which are exposed to the surface and which form a portion of the electrical conduction path is limited.
Therefore, the number of the particles exposed to the surface of the collector can be increased by roughening the surface of the collector bonded to the other collector or the electrode plate to increase the surface area thereof, and as a result, the initial ESR value of an electric double-layer capacitor is decreased.
Known techniques with the above-described way of thinking introduced thereinto includes
(1) a process for producing an electric double-layer capacitor
which is disclosed in Japanese Patent Application Laid-open No. 3-203215 by Takeda, wherein the surface of a collector is roughened by the abrasion suing a sandpaper; and
(2) a process for producing an electric double-layer capacitor
which is disclosed in Japanese Patent Application Laid-open No. 60-175408 by Okamoto, wherein when a gasket is fused to a collector using a ultrasonic process, the rugged shape formed in a horn of a ultrasonic fusing machine is transferred to the surface of the collector.
However, in the process in the item (1), it is necessary to add a roughening step, which is not preferable from a step-reducing point. In the process in the item (2), it is unnecessary to add a step, but in an electric double-layer capacitor of a thinner type in which a connector has a reduced thickness of 100 xcexcm, which is a recent tendency, it is extremely difficult to form a ruggedness in the surface of the collector by this producing process.
In either case of the items (1) and (2), voids are formed in the collector interface due to the fact that the surface of the collector has been roughened and for this reason, it is impossible to improve the long-term reliability of the ESR characteristic.
Accordingly, it is a general object of the present invention to provide an electric double-layer capacitor and a process for producing the same, wherein the problems associated with the above-described conventional techniques can be solved, whereby a low ESR value can be realized at the initial stage of the service of the capacitor and after the service of the capacitor for a long period.
It is a particular object of the present invention to provide an electric double-layer capacitor and a process for producing the same, wherein the contact state of an interface between basic cells constituting a cell structure is good at an initial stage of production of the capacitor and after the service of the capacitor for a long period.
It is another particular object of the present invention to provide an electric double-layer capacitor and a process for producing the same, wherein the contact state of an interface between a collector included in a basic cell and a conductive bond layer is good at an initial stage of production of the capacitor and after the service of the capacitor for a long period.
To achieve the above object, according to one aspect and feature of the present invention, there is provided an electric double-layer capacitor, comprising a cell structure formed by laminating basic cells in series one on another, each of the basic cells including a pair of collectors, a separator disposed between the collectors, gaskets disposed at peripheral edges of the pair of collectors and the separator to define a pair of closed spaces, and a pair of polarizable electrodes which are disposed in the closed spaces defined by the collectors, the separator and the gaskets, respectively and which contain an electrolytic solution, the basic cells being laminated one on another with a back side of each of said collectors opposed to the polarizable electrode serving as a connection surface; a lead terminal/electrode plate assembly having electrode plates bonded to the collectors located at opposite ends of the cell structure through conductive bond layers; and at least one of interfaces between the collectors included in the cell structure, which has a surface shape based on a rugged surface shape of the surface of the polarizable electrode contacting with and opposed to at least one of the two collectors forming the interface.
With the above arrangement, the interface between the collectors has such rugged shape and hence, has an increased contact area, as compared with a case when the rugged shape is not formed. Therefore, the number of electrical conduction paths formed in the interface is increased and as a result, the initial ESR value of the electric double-layer capacitor having the above-described arrangement is decreased.
In addition, the surface shape of the interface is a rugged shape which is derived from the moderate loosening of the rugged surface shape of the polarizable electrode by the elasticity of the collector and which has a relatively gentle angle of gradient, as is a sine wave. Thus, recesses on the bonded surface of the collector are in contact with the conductive bond layer opposed thereto without creation of voids. Therefore, the electric double-layer capacitor having such arrangement has an ESR characteristic having a high long-term reliability.
To achieve the above object, according to another aspect and feature of the present invention, there is provided an electric double-layer capacitor a basic cell including a pair of collectors, a separator disposed between said collectors, gaskets disposed at peripheral edges of said pair of collectors and said separator to define a pair of closed spaces, and a pair of polarizable electrodes which are disposed in said closed spaces defined by said collectors, said separator and said gaskets, respectively and which contain an electrolytic solution, a lead terminal/electrode plate assembly including electrode plates bonded to said basic cell through conductive bond layers, with at least one of backs of the collectors of the basic cell, which are opposed to the polarizable electrodes, serving as a connection surface, and the surface of at least one of said collectors forming an interface with said conductive bond layer having a surface shape based on the rugged surface shape of the polarizable electrode contacting with said collector.
With the above arrangement, the interface between the collector and the conductive bond layer included in the electric double-layer capacitor has such rugged shape and hence, has an increased contact area, as compared with a case when the rugged shape is not formed. Therefore, the number of electrical conduction paths formed in the interface is increased and as a result, the initial ESR value of the electric double-layer capacitor having the above-described arrangement is decreased.
In addition, the surface shape of the interface is a rugged shape which is derived from the moderate loosening of the rugged surface shape of the polarizable electrode by the elasticity of the collector and which has a relatively gentle angle of gradient, as is a sine wave. Thus, recesses on the bonded surface of the collector are in contact with the conductive bond layer opposed thereto without creation of voids. Therefore, the electric double-layer capacitor having such arrangement has an ESR characteristic having a high long-term reliability.
To achieve the above object, according to a further aspect and feature of the present invention, there is provided a process for producing an electric double-layer capacitor, comprising the steps of (A) putting a collector on a flat plate and a frame-shaped gasket element having an opening into abutment against each other to define a recess with a bottom surface formed by the collector and sides formed by the inner sides of the opening in the gasket element, (B) disposing a polarizable electrode impregnated with an electrolytic solution on the bottom surface of the recess, (C) placing two intermediate products fabricated through the steps (A) and (B) and a flat plate-shaped separator, so that end faces of the gasket elements forming the openings in the intermediate products are put into abutment against peripheral edges of thickness-wise opposite surfaces of the separator, (D) thermally fusing the two gasket elements included in an assembly resulting from the step (C) integrally to each other under a pressure lower than the atmospheric pressure and equal to or higher than 67 Pa to form a gasket, and covering a side of the separator with the gasket, thereby forming a basic cell including the two intermediate products and the separator integrally bonded to each other, (E) laminating a plurality of the basic cells formed in the above manner in series one on another, so that backs of the collectors included in each of the basic cells contacting with the polarizable electrodes are bonded surfaces, thereby forming a cell structure, (F) putting the two collectors forming opposite ends of the cell structure into abutment against electrode plates included in a lead terminal/electrode plate assembly through conductive bond layers, (G) pressing the surface of a laminate fabricated at the step (F), which is in abutment against the collector, from outside the electrode plates included in the lead terminal/electrode plates.
In such producing process, a single basic cell may be used in place of the lamination of the basic cells to produce an electric double-layer capacitor.
In the producing process, the pressure in the closed spaces included in the basic cell is lower than the pressure outside the basic cell and hence, the surface of collector forming an outer side of the basic cell is formed into a rugged shape based on a rugged shape formed by the surface of the polarizable electrode contacting with such collector. Thus, the basic cell including the collector having the rugged surface shape is produced without addition of a new step for forming the ruggedness on the surface of the collector. In the electric double-layer capacitor produced using plurality of basic cell or the single basic cell, the contact area of the interface between the collectors in the former case or the interface between the collector and the conductive bond layer in the latter case is larger than that in the electric double-layer capacitor produced in the conventional producing process. Therefore, the number of electrical conduction paths formed in the interface is increased and as a result, the initial ESR value of the electric double-layer capacitor having the above-described arrangement is decreased.
In addition, the surface shape of the collector is a rugged shape which is derived from the moderate loosening of the rugged surface shape of the polarizable electrode by the elasticity of the collector and which has a relatively gentle angle of gradient, as is a sine wave. This shape is largely different from a surface shape having an acute ruggedness and provided, for example, by rubbing of the surface by a sandpaper. Thus, recess on the bonded surface of the collector is brought into contact with the conductive bond layer opposed thereto or the other collector without creation of voids. Therefore, the electric double-layer capacitor having the above-described arrangement has an ESR characteristic having a high long-term reliability.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.